<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        .demo1 {
            height: 100px;
            background-color: red;
            margin-bottom: 100px;
        }
        .demo2 {
            height: 100px;
            background-color: red;
            margin-top: 100px;
            width: 100%;
            /* display: inline-block; */
            float: left;
        }
    </style>
</head>
<body>
    <div class="demo1"></div>
    <div class="demo2"></div>
</body>
</html>